Exterior wood substrates such as furniture, sheds, fences and other garden woods, are often surface-coated with weatherproofing coating compositions in order to prevent or delay weathering and to improve appearance. Aqueous coating compositions with acrylic resins as binder are often used for such coatings. Such acrylic resin based coating compositions typically have a low concentration of resin, usually less than 20 wt %, often less than 10 wt % or even less than 5 wt %, based on the total weight of the coating composition. The total solids content of such coating compositions is also relatively low, often below 25 wt % or even 15 wt %. As a result of the low resin and solids concentration, the viscosity of such coating compositions is relatively low. Thickeners are generally used to maintain can stability, i.e. to keep any inorganic particulate ingredients such as pigments and extenders, in suspension. The viscosity of such low resin, low solids coating compositions, also with thickeners, is usually below 0.5 Pa·s, often below 0.2 Pa·s or even below 0.1 Pa·s (as measured according to ISO 3219 at 23° C. and 100 s−1 shear rate).
Aqueous coating compositions are typically applied to substrates as a liquid by brush or by roller or as an aerosol by spray. For low viscosity coating compositions, brush or roller application typically results in dripping of the coating composition from the applicator (brush or roller) or from the substrate. Spray coating easily results in overspray.
It is known to apply aqueous coating compositions as a foam, in particular for coating compositions with a high solids content.
In U.S. Pat. No. 4,376,142 for example is disclosed foam application of a prime coating for fiberboard. An aqueous coating composition comprising clay, polyvinyl acetate latex, and a foaming agent is foamed and applied as a prime coat and then heated to have the foam collapsed and the coating dried. The latex coating compositions of U.S. Pat. No. 4,376,142 have very high solids content: typically 3 to 5 parts by weight of latex and 49 to 61 parts by weight of clay.
In U.S. Pat. No. 6,528,114 is disclosed a method for applying a substantially water and weather proof coating to a recessed surface (i.e. formed by cracks, cuts, holes and the like) of a wooden object by applying a foamed coating composition to the recessed surfaces. The foam subsequently collapses into a fluid that penetrates and seals the contacted wood surface. The coating compositions of U.S. Pat. No. 6,528,114 may be solvent-based or water-based. As suitable water-based coating composition is mentioned a coating composition comprising 35-45 wt % modified alkyd resin.
In JP2014132069 is disclosed a method for smoothly and uniformly coating wood surfaces by applying an aqueous coating composition as a foam. The aqueous coating composition of JP2014132069 is a composition with a relatively high solids content. It comprises 20-70 wt % solids of a synthetic resin or a natural oil or fat, a thixotropic agent, and 3 to 10 wt % of a lubricant such as for example polyethylene-based spherical particles, silicone powder, polymer powders or waxes. The coating composition may further comprise color pigments (up to 20 wt %) and 10-70 wt % of an inorganic extender. Exemplified coating compositions have solid contents in the range of from 50 to 70 wt %, and are either alkyd-resin based coatings or acrylic resin based plaster type coating compositions with high amounts of inorganic extenders.
For aqueous coating compositions with low solid content such as are typically used for decorating and weather-proofing exterior wood surfaces such as sheds, fences and other garden woods, there is a need for a more convenient way of application, also for the non-experienced painter, that results in a dried coating of acceptable appearance.